There have been conventionally used a flame rod as a typical flame detecting means using a flame conductivity in a combustion. The flame rod is placed in contact with flame produced on a burner. When a voltage is applied between the flame rod and the burner, a current flows between them owing to the presence of charged particles (ions and electrons) in the flame. The current is dependent on the conditions of combustion such as input rate and air-fuel ratio. The typical abnormal combustion caused by oxygen deficiency, abnormal air-fuel ratio and other factors reduces the current. Examples of such abnormal combustion detection using the flame rod may be found in U.S. Pat. Nos. 4,245,977 and 4,710,125.
This flame detection has a disadvantage described below. When combustion air contains a small amount of organic silicone compounds which is volatilized from a hair spray for example, an insulating silicon oxide is formed on surfaces of both the flame rod and the burner. As a result, the current is reduced due to its insulating property in spite of no ill effects of the silicone compounds on combustion. On the other hand, the abnormal combustion also reduces the current, as described above. These facts indicate that the conventional flame detection using a current is not able to distinguish whether the decrease in the current is due to the formation of the silicon oxide or is due to abnormal combustion. Therefore, when the current is reduced to some extent, combustion must be forcibly stopped to keep safety combustion even if combustion containing a small amount of silicone compounds is normal.
The conventional apparatuses for flame detection which are able to detect flame even under the conditions of combustion containing a small amount of organic silicone compounds are disclosed in the Japanese Pat. Laid-Open Nos. 6-101834 and 6-213432.
JP 6-101834 discloses a combustion apparatus comprising a flame rod where a portion of the surface of the flame rod in contact with the flame is grooved. This patent describes that the insulating silicon oxide is not formed on the groove because silicone compounds cannot reach the groove. Therefore, the current can flow through the groove.
JP 6-213432 discloses another combustion apparatus comprising a flame rod having a supplementary rod fixed at the portion contacting the flame. The supplementary rod is inferior in thermal stability with respect to the flame rod. This patent describes that the supplementary rod has a cracked surface due to its inferior thermal stability and that the freshly cracked surface on which the silicon oxide is not formed can be used again. Therefore, the current can flow through the cracked surface.
The conventional flame rods described above are effective only when the insulating silicon oxide is formed on the surface of the flame rod. However, since the silicon oxide is also formed on the surface of the burner, the conventional flame rods are ineffective when the insulating silicon oxide is formed on the surface of the burner.